Known air impact molding machines generally comprise a lower table, with a pattern plateholder arranged thereon, a box positioned over the latter and a frame provided on the box, all of which elements may be axially and telescopically coupled and uncoupled as appropriate to form a tight block with the pattern lying on its bottom where it can at any time receive through an upper diffuser the amount of sand needed to form the mold and at some other time be acted on by a bell-shaped ejector that can furnish the sand with the necessary air impact to compact the same and definitely form the mold.
Therefore and in the light of the above, the bell-shaped ejector and the sand supplying hopper must alternatively face the operative block wherein the actual molding takes place, which entails that such elements must be able to move with respect to the packet, and the pattern plateholder must also move so that the packet may be provided with each new pattern and the sand mold formed therein may be removed.
There are machines wherein the bell-shaped ejector and the sand hopper are established on a slide that can move crosswise and take up two working positions, viz. either with the operative molding block axially facing the bell-shaped ejector, where the sand hopper receives a new amount, or with the said sand hopper actually facing the molding block, whilst the bell-shaped ejector remains on one side and is at rest.
In another type of air impact molding machines, the bell-shaped ejector and the sand hopper are established on a revolving shaft that defines their above-mentioned two positions with respect to the operative molding block.
The pattern plateholder, in some existing machines is assembled on a slide that may move in opposite directions and in the machines of another type it is assembled on a revolving shaft, so that the plates at all events have two supports, one taking part in the operative molding block, whereas the other one allows the simultaneous removal of the sand mold which has already been formed and the appropriate substitution of the pattern.
In order to discharge the bell-shaped ejector, conventional machines of this type are fitted with a short cylindrical nozzle just above the deflector which normally seats a diaphragm located in an imaginary plane extending at the right angle to the ejector axis, which diaphragm is kept pressed against its seating, disconnecting the ejector and the molding area, through the pressure medium entering a chamber provided for such purpose on the diaphragm, so that when such pressure, which is greater than the pressure in the ejector, is ceased, the diaphragm is suddenly opened and so does the ejector to suddenly discharge onto the molding area.
This conventional system, however, implies that the ejector and the diffuser must be connected peripherally and with a double bend, so that the air first of all flows down the ejector peripheral area, then up towards the diaphragm and then down again through the diffuser, such a labyrinthine path entailing, on the one hand, a loss of load and, on the other hand, that the air impact will mostly take place at the central area of the sand mass constituting the mold, such air impact being rather smaller at the peripheral areas of the mold, especially at the corners, where the mold should be the hardest since those surfaces will serve to handle the same.